Ultrawide bandwidth (UWB) signals allow large amounts of data to be sent very rapidly at very low power. The UWB signals have their energy spread over a large frequency band, which significantly reduces the interference on any particular lesser frequency band. Nevertheless, it is desirable to keep low both the total power spectral density of a transmitted UWB signal, as well as the highest peak of the UWB signal's power spectral density.
FIG. 1 is a block diagram of a basic ultrawide bandwidth (UWB) transmitter circuit that can be used in a UWB transceiver. As shown in FIG. 1, the transmitter circuit 100 takes a set of transmit data and provides it to a signal generator 110. The signal generator 110 generates a UWB signal that has the transmit data encoded on it and provides that encoded signal to an antenna 120 for transmission.
In the embodiment shown in FIG. 1, a UWB antenna may be used, such as the one disclosed in U.S. Pat. No. 6,590,545 to McCorkle, entitled “Electrically Small Planar UWB Antenna Apparatus and System Thereof.” However, alternate embodiments can use different antenna designs.
In order for a UWB transceiver to function, it must generate signals at a very high frequency. In one proposed implementation of a UWB transceiver, signals up to nearly 10 GHz must be generated. However, many ways of generating appropriate UWB signals at such high frequencies introduce undesirable harmonics into the signals, which in turn can cause spikes (referred to as spectral lines) in the power spectral density of the transmitted signal.
One particular cause of undesirable harmonics comes from signal leakage in non-ideal circuit elements. Such leakage currents and voltages can introduce periodic interference or noise into a transmitted signal. And any periodic noise in a transmitted signal can produce undesirable spectral lines in that signal.
This can cause problems in the operation of UWB devices since under current FCC regulations the transmit power for UWB devices is limited by the power spectral density (PSD) of the transmitted signal. FIG. 2 is a graph showing the shape of power spectral density limits currently put in force by the FCC for the operation of UWB devices. They impose a maximum limit in the range of about 3-10 GHz, with increasing drops outside of this range.
The FCC will not allow any portion of the power spectral density of a UWB signal to rise above the limits it imposes. As a result, if a UWB signal includes a spike in its power spectral density, the total transmission power of the UWB signal must be reduced until that spike falls below the FCC's power limits. This can significantly reduce the total signal power of the transmitted signal. Thus, any spectral lines (i.e., spikes in the power spectral density) in a signal transmitted by a UWB device can reduce the effectiveness of that device. It is therefore very important that the power spectral density of the signal output by the signal generator 110 and transmitted by the antenna 120 be strictly controlled, and be as even as possible.
Accordingly, it would be desirable to reduce the magnitude of any spikes in the power spectral density of any UWB signal generated by a UWB transceiver.